Our objective is to elucidate the hereditary control of organelle structure and function by studying Mendelian (nuclear) and non-Mendelian (presumably organelle) mutations affecting chloroplast and mitochondrial functions. The green alga, Chlamydomonasreinhrdtii, is a model system which allows isolation and characterization of mutations lacking specific chloroplast functions (acetate requirers) and muations with altered mitochondrial funtions (obligate photoautotrophs). In no other species can mutations of the chloroplast and mitochondrion be investigated simultaneously to determine the genetic interrelationship of both organelles to the nucleus and to each other. One well-characterized non-Mendelian genetic syste exhibiting uniparental (UP) inheritance is well known in C. reinhardtii and resides in chloroplast DNA. Recently we have identified and begun characterization of a second non-Mendelian genetic system which shows biparental inheritance and appears to be mitochondrial. Our attention has also focused on two classes of mutations affecting the chloroplast, those controlling the formation and function of chloroplast ribosomes and those involved in porphyrin biosynthesis.